Isotopic anomalies of Ne, Xe, and C in meteorites. II. Interstellar diamond and SiC: Carriers of exotic noble gases
Abstract
Continuing our efforts to find the carriers of presolar noble gases, we isolated 15 separates from the Murray C2 chondrite on the basis of grain size and chemical resistance, and analyzed them by mass spectrometry for Ne and Xe. The abundant (570 ppm) Cδ diamond, carrier of Xe-HL and Ne-A1, 2, was efficiently removed by a new type of colloid separation. The rare component Xe-S was enriched ~ 17-fold over the best previous sample; its carrier Cβ appears to be SiC, of much larger size than Cδ (mean diameter ~0.12 μm vs. 0.0026 μm) and lower abundance (~2 ppm), but likewise showing a sharply peaked log-normal size distribution. If these distributions reflect grain growth from circumstellar gas shells, then their near-perfect shape suggests minimal alteration of the grains by secondary processes such as fragmentation or erosion, and hence a short residence time in interstellar space. A third component, Ne-E(H), was enriched ~ 9-fold over the best previous sample in an acid-insoluble residue of SiC, Cδ, and chromite, which was left when a coarse spinel fraction was dissolved. This SiC (~7 ppm), differing from the Cβ-SiC mentioned above, appears to be Cɛ, the carrier of Ne-E(H). The data obtained thus far show that both diamond (Cδ) and amorphous carbon (Cα, Cθ) occur as interstellar grains, along with SiC (Cβ, Cɛ). Crystalline graphite, though long predicted as a stable stellar condensate at high C/O ratios, has not been found. Each noble-gas component is associated with only one carbon allotrope or compound, which suggests that carbon condensation in a given star is strongly selective.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- May 1988
- DOI:
- 10.1016/0016-7037(88)90277-3
- Bibcode:
- 1988GeCoA..52.1235M
- Keywords:
-
- Diamonds;
- Isotopic Enrichment;
- Meteoritic Composition;
- Rare Gases;
- Silicon Carbides;
- Carbon;
- Chondrites;
- Neon;
- Xenon;
- METEORITES;
- ISOTOPES;
- ANOMALIES;
- NEON;
- XENON;
- CARBON;
- RARE GASES;
- DIAMOND;
- SAMPLES;
- METEORITE;
- MURRAY;
- SPECTROMETRY;
- ENRICHMENT;
- PROCEDURE;
- GRAPHITE;
- LABORATORY STUDIES